CNT have excellent properties of electric characteristics, mechanical strength and others, and studies and developments thereof as ultimate new materials are being made energetically. CNT are produced in various methods of a laser vaporization method, an arc discharge method, a chemical vapor deposition method (CVD method), etc. At present, however, they are produced only as a mixture morphology of metallic CNT and semiconducting CNT in any production methods.
In practical use, the properties of either one only of metallic or semiconducting CNT are needed in many cases, and therefore, the studies of separating and purifying metallic or semiconducting CNT alone from a CNT mixture are important ones that are desired to be solved as quickly as possible.
Heretofore, there are given some reports relating to separation of metallic CNT and semiconducting CNT from each other; however, all these involve problems in industrial production of metallic CNT and semiconducting CNT. The problems are as follows: (1) As the process includes complicated steps, it could not be automated. (2) The process takes a long time. (3) The process is not applicable to mass-production. (4) The process requires expensive equipment and chemicals. (5) Only either metallic CNT or semiconducting CNT could be obtained. (6) The collection rate is low.
For example, there are known a method of electrophoresing CNT dispersed with a surfactant, on a microelectrode (Non-Patent Reference 1); a method of using amines as a dispersant in a solvent (Non-Patent References 2, 3); and a method of selectively combusting semiconducting CNT with hydrogen peroxide (Non-Patent Reference 4); however, these could not still solve the problems of those mentioned above, especially in that the final product is limited to metallic CNT alone and the collection rate thereof is low.
There are known a method of separating semiconducting CNT by dispersing a mixture of semiconducting CNT and metallic CNT in a liquid, then selectively binding the metallic CNT to particles, and removing the metallic CNT bound to the particles (Patent Reference 1); a method of obtaining semiconducting CNT by treating CNT with a nitronium ion-containing solution followed by filtration and heat treatment to remove the metallic CNT from CNT (Patent Reference 2); a method of using sulfuric acid and nitric acid (Patent Reference 3); a method of obtaining semiconducting CNT by selectively moving and separating CNT through application of an electric field thereto followed by restricting the electroconductivity range (Patent Reference 4).
These could not still solve the problems of those mentioned above, especially in that the final product to be obtained is limited to semiconducting CNT alone and the collection rate thereof is low.
There is known a method of separating CNT dispersed with a surfactant into metallic CNT and semiconducting CNT through density-gradient ultracentrifugation (Non-Patent Reference 5). The method involves some problems in that it requires an extremely expensive instrument of ultracentrifuge and takes a long time for ultracentrifugation, scaling up the ultracentrifuge itself is limited and a plurality of ultracentrifuges must be disposed in parallel, and therefore, automated treatment is difficult.
There is known a method of separation through ion-exchange chromatography by producing a CNT-nucleic acid composite comprising CNT bound to nucleic acid molecules (Patent Reference 5). However, this is problematic in that it requires an expensive synthetic DNA and the collection rate and the purity are not good since the separation accuracy is not so high.
There is known a report of trying separation of metallic and semiconducting CNT from each other by controlling the pH and the ionic intensity of a CNT solution dispersed with a surfactant to cause a different degree of protonation depending on the type of CNT, followed by applying an electric field to the resulting solution for the intended separation (Patent Reference 6). However, the method requires a step of pretreatment with a strong acid for pH and ionic intensity control of the suspended nanotube mixture prior to separation, and therefore, severe process control for the step is inevitable and finally, in addition, the separation of metallic and semiconducting CNT from each other could not be attained (Patent Reference 6, Example 4).
Also known is gellation of CNT themselves by the use of an ionic liquid (Patent Reference 7, Patent Reference 8); however, these are for the purpose of obtaining a gel of CNT themselves for enhancing the dispersibility of CNT and for processing CNT, not going any further.
As described in the above, all the conventional methods could not solve the above-mentioned problems, and it is desired to develop a method for separating metallic CNT and semiconducting CNT from CNT based on a novel idea.
The present inventors have made a start of a novel method of separation of metallic CNT and semiconducting CNT that differs from conventional methods, and have completed a invention mentioned below (Patent References 9, 10). The invention is as follows: A “CNT-containing gel” which is CNT previously dispersed and isolated in a gel is prepared, and an electric field is applied to the CNT-containing gel for gel electrophoresis whereupon only metallic CNT move but semiconducting CNT do not move at all as kept adsorbed by the original gel, and the semiconducting CNT and the metallic CNT are thereby separated from each other. This method is extremely excellent in that both metallic CNT and semiconducting CNT are obtained and, in addition, the collection rate is high and the separation is attained within a short period of time, and further, using inexpensive equipment, the method is simple and enables large-scale mass-production (Patent Reference 9).
The above method has attained separation of metallic CNT and semiconducting CNT according to an electric separation means of applying an electric field to a CNT-containing gel. Apart from this, the present inventors have found that the separation could be possible by a physical separation means not by the electric separation means (Patent Reference 10). For example, when a CNT-containing gel is centrifuged, then the gel is crushed and is separated into a squeezed solution and the gel residue. In this stage, metallic CNT is fractionated in the solution fraction and semiconducting CNT is in the gel fraction. This method enables the separation in more simplified equipment than in the electrophoresis method, and facilitates scaling up and automation directed to mass-scale production.
However, in this method, CNT adsorbed by the gel must be taken out of the gel and collected for utilizing them for various purposes. Heretofore, for removing the gel from CNT adsorbed by gel, an operation of dissolving the gel by heating followed by separating the solution through centrifugation into a precipitate of CNT and the supernatant containing the dissolved gel must be repeated; and further, for completely removing the minor amount of the gel still remaining around the CNT, an operation of heating the system in an acid-containing solution to decompose it must also be repeated, and the process is extremely complicated (Non-Patent Reference 6).    [Patent Reference 1] JP-A 2007-31238    [Patent Reference 2] JP-A 2005-325020    [Patent Reference 3] JP-A 2005-194180    [Patent Reference 4] JP-A 2005-104750    [Patent Reference 5] JP-A 2006-512276    [Patent Reference 6] JP-A 2005-527455    [Patent Reference 7] JP-A 2004-142972    [Patent Reference 8] JP-A 2006-282418    [Patent Reference 9] JP-A 2008-285387    [Patent Reference 10] Japanese Patent Application No. 2007-318302    [Non-Patent Reference 1] Advanced Materials 18, (2006) 1468-1470    [Non-Patent Reference 2] J. Am. Chem. Soc. 127, (2005) 10287-10290    [Non-Patent Reference 3] J. Am. Chem. Soc. 128, (2006) 12239-12242    [Non-Patent Reference 4] J. Phys. Chem. B 110, (2006) 25-29    [Non-Patent Reference 5] Nature Nanotechnology 1, (2006) 60-65    [Non-Patent Reference 6] Nano Letters 9, (2009) 1497-1500